[llvm] [X86] Adding lowerings for vector ISD::LRINT and ISD::LLRINT (PR #90065)

[Florian Hahn via llvm-commits]

fhahn wrote:

It looks like this change is causing different results in some cases when using `@llvm.lrint.v4i64.v4f32`.

For

```
define i32 @foo(ptr %dst, <4 x float> %in) {
entry:
  %0 = call <4 x i64> @llvm.lrint.v4i64.v4f32(<4 x float> %in)
  %1 = trunc <4 x i64> %0 to <4 x i32>
  store <4 x i32> %1, ptr %dst, align 4
  ret i32 0
}
```


we now generate the assembly below, which does the conversion to signed double word.

```
_foo:                                   ## @foo
        cvtps2dq        xmm0, xmm0
        movupd  xmmword ptr [rdi], xmm0
        xor     eax, eax
        ret
```

Before this change, we generated the assembly below, which does the conversion to signed quad word, then truncating the result

```
_foo:                                   ## @foo
        movaps  xmm1, xmm0
        shufps  xmm1, xmm0, 85                  ## xmm1 = xmm1[1,1],xmm0[1,1]
        cvtss2si        rax, xmm1
        cvtss2si        rcx, xmm0
        movaps  xmm1, xmm0
        unpckhpd        xmm1, xmm0                      ## xmm1 = xmm1[1],xmm0[1]
        cvtss2si        rdx, xmm1
        shufps  xmm0, xmm0, 255                 ## xmm0 = xmm0[3,3,3,3]
        cvtss2si        rsi, xmm0
        movd    xmm0, esi
        movd    xmm1, edx
        punpckldq       xmm1, xmm0              ## xmm1 = xmm1[0],xmm0[0],xmm1[1],xmm0[1]
        movd    xmm0, ecx
        movd    xmm2, eax
        punpckldq       xmm0, xmm2              ## xmm0 = xmm0[0],xmm2[0],xmm0[1],xmm2[1]
        punpcklqdq      xmm0, xmm1              ## xmm0 = xmm0[0],xmm1[0]
        movdqu  xmmword ptr [rdi], xmm0
        xor     eax, eax
        ret
```

I might be missing something, but I think for inputs like 2^33 new codegen will overflow, while the second won't.

See https://llvm.godbolt.org/z/E7W89q59M for a comparison `main` vs `18.x`

https://github.com/llvm/llvm-project/pull/90065